The present invention relates to a semiconducting gas sensor in accordance with the preamble of Patent claim 1, a gas sensor system, and a method of gas analysis using a semiconducting gas sensor.
The present invention relates to a semiconducting gas sensor, and to a method of gas analysis using a semiconducting gas sensor,
In some fields, gas analysis is of great importance. For example, in the combustion of fossil fuels, carbon monoxide and nitrous oxides or NOx, are produced, which may then be converted to O3. The damage to the environment caused by these substances is considerable. For this reason it is highly imperative that exhaust gases produced by internal combustion engines be analyzed, with an eye to reducing their emission of pollutants.
One possibility for gas analysis is presented by semiconducting gas sensors, in which a gas-sensitive metallic oxide layer, such as SnO2, is brought to a specific measuring temperature. By measuring the electrical resistance of the gas-sensitive layer at a specific temperature, the gas concentrations, for example of CO, NOx, or O3, can be determined.
The article by B. Ruhland, et al., xe2x80x9cGas-Kinetic Interactions of Nitrous Oxides with SnO2 Surfacesxe2x80x9d, Sensors and Actuators B 50 (1998) pp 85-94, discusses a semiconducting gas sensor of this type. In this known gas sensor, a thin layer of SnO2 is placed on a heating structure. An SiO2 layer separates a heating element from the gas-sensitive SnO2 layer. The heating structure with the gas-sensitive layer is arranged on a Si3N4 membrane, which is then placed over a silicon substrate. In the measurement process, the gas that is to be analyzed flows over the sensor element. The bombardment with the gas components to be analyzed can also be accomplished via diffusion.
In the measurement of gases comprising several components, the problem arises that the effects of the individual gas components may become superimposed in the measuring signal. For example, at a measuring temperature of 400xc2x0 C., a bombardment of the gas-sensitive layer with CO or NO leads to a reduction in the electrical resistance of the gas-sensitive layer, while a bombardment with NO2 at this temperature results in an increase in electrical resistance. Furthermore, the contact between the gas-sensitive layer and ozone results in increased resistance. For this reason, the individual concentrations in the gas mixture frequently cannot be precisely determined.
One possibility for solving this problem consists in providing an arrangement comprising several sensors having different measuring temperatures. While a considerable degree of NO2 sensitivity is present even at relatively low temperatures of 150xc2x0 C. to 250xc2x0 C., a suitable measuring temperature for CO, for example, lies between 350xc2x0 C. and 450xc2x0 C. The arrangement with the whole sensor array, however, is expensive, and thus associated with relatively high costs.
Another approach to solving the problem involves obtaining comparative sets of data for defined individual gases and gas mixtures at various temperatures via experimentation. To this end, the above-mentioned publication provides for a bombardment of several sensor elements with individual gas components at defined concentrations, in order to determine the behavior of electrical resistance, as a function of temperature. With the resistance behavior determined in this manner, it is then possible to analyze a gas mixture comprised, for example, of CO and NO2 using two sensors, wherein one sensor is operated at 200xc2x0 C. and one sensor is operated at 400xc2x0 C. One disadvantage of this process is that it allows only very simple gas mixtures to be analyzed. Furthermore, interactions between the gases are not taken into account.
In addition, the high O3 sensitivity disrupts the measurement process significantly. In many cases, the ozone sensitivity outweighs all other effects. For example, with ozone concentrations that are higher than 100 ppb the measuring signal can be interpreted only as an ozone signal.
It is thus one object of the present invention to create a semiconducting gas sensor and a gas sensor arrangement that is suitable for analyzing a gas or gas mixture comprising a number of components, such as, for example, ozone and that can be produced simply and cost-effectively. Furthermore, a method of gas analysis is to be provided, which will enable the analysis of a gas or gas mixture comprising a number of components via semiconducting sensors.
This and other objects and advantages are achieved by the semiconducting gas sensor according to the invention, which comprises a gas-sensitive layer, whose electrical conductivity can be altered via contact with a gas, a heating apparatus for heating the layer to a defined measuring temperature, contact electrodes for measuring the electrical resistance or the electrical conductivity of the gas-sensitive layer, and a chamber in which the gas sensitive layer is positioned. The chamber can be sealed from the outside; and the volume of the chamber is small enough that at least one component of the gas or gas mixture is largely exhausted via conversion, within a predetermined measuring interval, for example on the gas-sensitive layer.
In this manner, the disruptive effects of ozone on the measurement process can be eliminated.
With the small chamber volume, individual components of the gas become converted during the measuring process, so that they do not contribute, or contribute only slightly, to the measuring signal. The remaining measuring signal is then no longer superimposed by the effects of the gas components that have already been converted, allowing the concentrations of the remaining components to be more easily determined. With the invention it is possible to determine the concentrations of different gas components in a gas mixture, without requiring a multitude of sensors operating at different temperatures, which require costly evaluation. In addition, the gas analysis can be accomplished within a relatively short period of time, with the chamber volume being dependent upon the type of gas to betanalyzed and the desired duration of the measuring interval.
Advantageously, the semiconducting gas sensor comprises a regulating device that enables the heating of the gas-sensitive layer in stages, thus allowing individual components of the gas mixture to be selectively converted at predetermined measuring temperatures. Preferably, the semiconducting gas sensor is produced using micromechanical technology, for example via Si technology. This enables a simple, cost-effective production, and a standard implementation of the sensor.
A platinum heating resistor, arranged in a meandering pattern, is preferably used as the heating device. The contact electrodes are preferably also made of platinum. This serves to produce increased temperature stability, while preventing mutual interference between the electrodes and the resistance material.
Advantageously, a passivating layer, comprised, for example, of SiO2, is positioned between the heater and the gas-sensitive layer and serves as an insulator. Specifically, a silicon substrate may be provided as the supporting material, along with a nitride membrane, which separates the heater from the substrate.
The gas-sensitive layer is preferably comprised of SnO2, however it can also be made of other metallic oxides such as WO3 and titanium oxide, or of organic materials such as phthallocyanine.
The semiconducting gas sensor is preferably designed to be suitable for measuring concentrations of CO, NO2, NO, and/or O3. The chamber is preferably a microchamber made, for example, of silicon. The chamber volume advantageously measures approx. 10 to 500 xcexcl, preferably 10 to 100 xcexcl, and most preferably approx. 40 xcexcl.
In accordance with a further aspect of the invention, a gas sensor system is provided, which comprises several semiconducting gas sensors as specified in the invention, along with an arrangement of regulated valves and lines for the inlet and outlet of gas. In this manner, it is possible to create redundancies, and to cost-effectively increase the lifespan of the system. In addition, a multitude of gas sensors may be used individually, allowing an improvement in measuring quality or precision to be achieved. Preferably, the semiconducting gas sensors are arranged in a parallel connection, wherein the valves may be controlled individually.
In accordance with a further aspect of the invention, a method of gas analysis using a semiconducting gas sensor is provided, which comprises the following steps: preparing a semiconducting gas sensor with a gas-sensitive layer in a sealable chamber; filling the chamber with a gas or gas mixture that is to be analyzed, and sealing the chamber; heating the gas-sensitive layer to a predetermined measuring temperature, and examining a measuring signal that is dependent, for example, upon the electrical conductivity or the ohmic resistance of the gas-sensitive layer, at a moment of measurement in which at least one gas component has been exhausted via conversion within the chamber so that it does not contribute, or contributes relatively slightly, to the measuring signal; and determining of the content of at least one remaining gas component from the remaining measuring signal.
With this process, gases or gas mixtures comprising several components, including ozone, may be easily analyzed, while the cost remains low, and the evaluation process implemented can be simple.
Advantageously, the measuring signal is used at least two different times during the measurement process to determine the components of the gas. For example, from the peak of the measuring signal and its subsequent decrease, the concentration of at least one component of the gas can be determined. Advantageously, the measuring temperature lies within a range of approx. 20xc2x0 C. to 550xc2x0 C., preferably within a range of approx. 50xc2x0 C. to 400xc2x0 C., and most preferably within a range of approx. 200xc2x0 C. to 400xc2x0 C. Therein, the heating process may be implemented gradually or in stages, with measurements being taken at different temperature stages. Most preferably, a semiconducting gas sensor as specified in the invention and/or a gas sensor system as specified in the invention are used in this process.